CVE-2026-98127

In the Linux kernel, the following vulnerability has been resolved:

smb/client: validate new EOF for insert range

smb3_insert_range() does not check if the new file size (i_size + len) is valid. This allows FALLOC_FL_INSERT_RANGE to bypass RLIMIT_FSIZE, exceed s_maxbytes, or produce a size outside the loff_t range.

Use check_add_overflow() to calculate the new EOF. Validate it with inode_newsize_ok() before modifying the file.

Reproducer, using a file on a CIFS mount:

bash -c 
	FILE=/mnt/cifs/repro

	trap  SIGXFSZ
	ulimit -f 3072		# RLIMIT_FSIZE = 3 MiB

	# A regular write is stopped at 3 MiB.
	dd if=/dev/zero of=$FILE bs=1M count=4 status=none
	stat -c size after write: %s $FILE

	# Insert 2 MiB into a 2 MiB file.
	truncate -s 2M $FILE
	fallocate -i -o 0 -l 2M $FILE
	stat -c size after insert: %s $FILE

Before this change, the regular write stops at the 3 MiB limit, but insert range grows the file to 4 MiB:

dd: error writing /mnt/cifs/repro: File too large
size after write: 3145728
size after insert: 4194304

After this change, insert range also fails at the limit and leaves the 2 MiB file unchanged:

dd: error writing /mnt/cifs/repro: File too large
size after write: 3145728
fallocate: fallocate failed: File too large
size after insert: 2097152

Weakness

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.

Affected Software

Potential Mitigations

• Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
• If possible, choose a language or compiler that performs automatic bounds checking.
• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid [REF-1482].
• Use libraries or frameworks that make it easier to handle numbers without unexpected consequences.
• Examples include safe integer handling packages such as SafeInt (C++) or IntegerLib (C or C++). [REF-106]
• Perform input validation on any numeric input by ensuring that it is within the expected range. Enforce that the input meets both the minimum and maximum requirements for the expected range.
• Use unsigned integers where possible. This makes it easier to perform validation for integer overflows. When signed integers are required, ensure that the range check includes minimum values as well as maximum values.
• Understand the programming language’s underlying representation and how it interacts with numeric calculation (CWE-681). Pay close attention to byte size discrepancies, precision, signed/unsigned distinctions, truncation, conversion and casting between types, “not-a-number” calculations, and how the language handles numbers that are too large or too small for its underlying representation. [REF-7]
• Also be careful to account for 32-bit, 64-bit, and other potential differences that may affect the numeric representation.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/92.html

References

• https://git.kernel.org/stable/c/1519dc88c87f5346dae0464d7d6da1b6bf1f6e8e
• https://git.kernel.org/stable/c/7c2c69045aa76ccaa7f16f1478eb312130ce4951
• https://git.kernel.org/stable/c/b7f2e33d9f5db7b8e6e5e0a5a93d0c5e31870356
• https://git.kernel.org/stable/c/bc7602b9082ef195d913c4c16fa9853dd262333e
• https://git.kernel.org/stable/c/f8cbfe3a3fcee7d991fd223f660f3e5df13221cf